This invention relates to a method for selectively removing the mass-91 isotope of zirconium (.sup.91 Zr) from natural zirconium, and more particularly to a method of selectively ionizing .sup.91 Zr atoms for removal by a charged collector.
Zirconium metal is vital to the nuclear power industry because of its low cross section for thermal neutron absorption. This property, combined with its toughness and corrosion resistance, makes zirconium almost ideal for nuclear reactor core components, such as pressure tubes and calandria, and for the cladding of nuclear fuel bundles. However, reactor operation could be improved if the thermal neutron absorption cross-section of zirconium in reactor components were reduced further. For example, the thickness of pressure tubes could be increased to extend their service lifetimes without increasing, and perhaps decreasing, their neutron absorption. It has been recognized for many years that depleting or removing entirely one of the isotopes or zirconium would reduce its neutron cross section. Zirconium has five isotopes, of atomic mass 90, 91, 92, 94, and 96. The mass-91 isotope has a natural abundance of just above 11%, but it accounts for 75% of the thermal neutron absorption cross section in natural zirconium. By reducing the concentration of .sup.91 Zr to 3%, the neutron cross section of the denatured zirconium would be less than half that of the natural metal.
Mass 91 is an "interior" isotope and thus cannot easily be stripped with ordinary techniques (e.g., diffusion). However, a method that has been developed to separate metal isotopes is the atomic vapor laser isotope separation (AVLIS) process wherein a beam of neutral metal atoms is irradiated by tunable lasers to ionize atoms of a desired isotope of the metal. The ionized atoms are attracted to an electrically charged extractor while the beam of neutral atoms continues unimpeded in its flow until it condenses on a collector plate.
Hackett et al., "The first ionization potential of zirconium atoms determined by two-laser fieldionization spectroscopy of high lying Rydberg series," J. Chem. Phys., Vol. 85, No. 6, Sep. 15, 1986, pp. 3194-3197, describes the determination of an accurate value for the first ionization potential (I.P.) of zirconium to be 53 506.0(3) cm.sup.-1. In a later publication by Hackett et al., "Pulsed single-mode laser ionization of hyperfine levels of zirconium-91," J. Opt. Soc. Am. B., Vol. 5, No. 12, December 1988, pp. 2409-2416, two-color mass-selective ionization of .sup.91 Zr in an atomic beam was demonstrated. Isotopic selectivity was obtained on the first of the three absorption transitions (.lambda..sub.1) leading to odd-parity energy levels (E.sub.1) in the 16000-19000 cm.sup.-1 range. They proposed that with proper choices of these .lambda..sub.1 transitions (between known energy levels of zirconium), an AVLIS process could be developed. By doing spectroscopic searches, some resonances were found for the .lambda..sub.2 transitions, and even-parity energy levels (E.sub.2) in the 35000-37000 cm.sup.-1 range were cataloged. Some ionization was caused by the non-resonant absorption at the E.sub.2 energy level of another photon at wavelength .lambda..sub.2, bringing the total absorbed photon energy above the ionization potential (IP) of 53506 cm.sup.-1.
Although Hackett et al. (1988) demonstrated on a microscopic scale that it would be possible to separate .sup.91 Zr by an AVLIS process, this publication does not disclose how such a process could be used with sufficient efficiency that a commercial operation could be carried out. The weakest step in a photoionization sequence is generally the addition of photon energy to ionize atoms from an E.sub.2 energy level. With respect to this step, Hackett et al. 1988 recognize that if a .sup.91 Zr atom is at a particular E.sub.2 energy level, for example at an energy level of 36068 cm.sup.-1, that the wavelength of a further absorbed photon must be .ltoreq.573.5 nm in order to ionize the atom. The 573.5 nm wavelength is, of course, deduced by subtracting the E.sub.2 energy level of 36068 cm.sup.-1 from the ionization potential of 53506 cm.sup.-1, and then taking the reciprocal thereof. The wavelengths used by Hackett et al. 1988 for the .lambda..sub.2 transitions are short enough so that they will cause some ionization, but with very low efficiency. The Hackett et al. 1988 publication does not disclose how ionization of .sup.91 Zr atoms can be accomplished efficiently, with resonant excitation by a photon of a .lambda..sub.3 wavelength, causing a transition from a E.sub.2 energy level to an autoionizing level.
Also, the radiative lifetime of .sup.91 Zr atoms at E.sub.2 energy levels and the .lambda..sub.3 transition cross sections are not addressed by Hackett et al. 1988. These parameters govern the laser requirements needed to produce ionization and thus impact the economic feasibility of an AVLIS process for the selective removal of .sup.91 Zr atoms.